In the prior art, as the mold for molding an optical part with a desired shape by heating and softening a glass or a plastic, followed by pressurization, a dice steel, a stainless steel or a hard alloy, has been used. However, these mold materials have the problem that they are short in life because of mold releasability between the mold and the material to be worked softened by heating, particularly in the case of a glass softened at a high temperature or the surface precision of the optical part after molding. A large number of mold materials for molding of an optical part for solving this problem have been proposed, and representative of them are Japanese Provisional Patent Publication No. 266321/1986, Japanese Provisional Patent Publication No. 50334/1988 and Japanese PCT Provisional Patent Publication No. 503058/1988.
Japanese Provisional Patent Publication No. 266321/1986 discloses a mold for glass molding comprising a mold itself constituted of electroconductive ceramics, having a mirror working applied on its molding surface, with the mold itself being heat generated through current passage, or a mold for glass molding comprising a coating comprising carbide or nitride of Si covered on the above molding surface. The mold for glass molding of the Publication is constituted of electroconductive ceramics and its mold itself is heat generated by current passage, whereby it has the advantage that the heated temperature on the molding surface of the mold is uniformized. Also, when a film comprising carbide or nitride is covered on the molding surface, as compared with a mold comprising a steel or a hard alloy of the prior art, an excellent feature can be obtained that the material to be worked softened by heating can become readily released from the mold. However, particularly when the material to be worked comprises a glass, it is required to be worked at a high temperature, and during such high temperature molding, the molding surface of the mold will react with the glass, whereby there are involved such problems that the surface of the glass after molding is susceptible to coarsening, particularly that it is difficult to obtain a molded article having equal surface precision to that of the molding surface of the mirror polished mold material, that the molded glass is liable to be colored, and that the glass after molding can be released from the mold with difficulty.
Japanese Provisional Patent Publication No. 50334/1988 discloses a mold for molding of an optical glass element comprising a metal nitride layer such as NbN, ZrN or AlN formed on a hard alloy.
The mold for molding of an optical glass element in the Japanese Provisional Patent Publication No. 50334/1988 is less in reactivity with the glass softened by heating as compared with the mold for molding comprising a steel or a hard alloy of the prior art, because the hard alloy does not directly contact with the glass, and excellent in mold releasability. However, there are involved such problems that the molded glass is liable to be colored, particularly that it is difficult to mirror polish the molding surface of the mold material and mold a glass to a glass surface having the surface precision of such mirror polishing, and that mold releasability is not still satisfactory because the reaction occurs between a glass and a nitride.
Japanese PCT Provisional Patent Publication No. 503058/-1988 discloses a method for using an Al.sub.2 O.sub.3 ceramics material, a ceramics material containing ZrO.sub.2 and/or HfO.sub.2 and a ceramics material comprising a mother material of tetragonal ZrO.sub.2 and Al.sub.2 O.sub.3, and/or Cr.sub.2 O.sub.3 as a press tool for molding a constituent material comprising a glass or a ceramics material containing a glass. In the same Publication, there is disclosed a method for using an oxide ceramics as a press tool for molding of a glass. However, this uses an oxide ceramics material such as Al.sub.2 O.sub.3 ceramics, ZrO.sub.2 and/or HfO.sub.2 ceramics, which is a material for tool of the prior art as the mold for molding of a glass. Accordingly, no investigation has been done about mold releasability between the glass and the mold for molding or the influence of the mold for molding on the glass, such as the reaction degree between the compositional components of the mold for molding and the glass or the molding surface precision of the glass. Also, all the oxide ceramics cannot be used for the tool for molding of a glass, but, for example, when a glass is molded by use of a mold of Al.sub.2 O.sub.3 ceramics or ZrO.sub.2 ceramics of the prior art, there are problems that the mold and the glass will be readily adhered, and that the surface precision of the glass molding surface after molding is inferior.